This invention relates to analysis of materials, and more particularly, relates to method and apparatus for the qualitative and quantitative determination of constituents of materials.
The spectral analysis of small particles of a specific material may be accomplished by introducing these particles directly into a plasma. The high temperature and high energy conditions found in a plasma may then cause the molecules and atoms to emit their characteristic spectra which can usually be readily identified.
Plasmas of excited gases may be generated by a variety of means including chemical flames, arc or spark discharges, inductively coupled radio frequency, and microwave-sustained gas discharges and may use support and/or excitation gases such as argon, helium, nitrogen, or oxygen.
Most plasma analysis techniques employ a carrier gas to carry a volatile particulate or gaseous sample to the plasma. However, use of a carrier gas to transport particles to and from the plasma usually results in certain shortcomings. More specifically, particulate fragments may recombine or condense on the conduit walls or may be adsorbed on the walls of the pyrolysis chamber and any transporting conduit. This usually results in a loss of material and "ghosting" or "memory effect" of materials, i.e. materials are adsorbed on the various sample transfer conduits and other interior surfaces and are introduced into the plasma during subsequent sample analysis which may result in spurious, erroneous analytical results. This loss of part of the sample, combined with an additional difficulty of maintaining reproducible pyrolysis temperatures and other conditions, makes it extremely difficult to obtain reproducible quantitative results.
Other problems may be associated with specific types of samples. As noted in U.S. Pat. No. 4,532,219, it is usually impossible to completely volatilize organic samples during pyrolysis without losing some portion of the sample to the walls of the pyrolysis apparatus and to any transfer conduits. The volatilization of a sample without the usual charring step is taught by "Simultaneous Determination Of 10 Elements In Waste Water, Plasma, and Bovine Liver By Inductively Coupled Plasma Emission Spectrometry With Electrothermal Atomization", Blakemore et al, Anal. Chem., 56, (1984) pp 1376-1379, but does not deal with organic samples.
Another technique that is common in the prior art is to aspirate and atomize or nebulize non-volatile material into a plasma. When materials are introduced into the plasma by aspiration, nebulization, atomization, or even directly more subtle problems may arise. For example, when an aerosol reaches a plasma, the aerosol may absorb energy from the plasma and as a result, may seriously affect plasma operating conditions and performance. In an extreme case, sample introduction may overload the plasma sufficiently to extinguish it entirely. A survey of electrothermal vaporization techniques for samples employed in atomic emission spectroscopy is found in "Electrothermal Vaporization for Sample Introduction in Atomic Emission Spectrometry", NG et al, Applied Spectroscopy Vol. 39, No. 4 (1985), pp 719-726.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for analysis of samples.